Terminations for high voltage (i.e., greater than about 1 kV) polymeric insulated electrical transmission cables are typically accomplished by cutting back an outer polymeric jacket, neutral conductors, a semiconductive layer, and a primary polymeric insulation layer to expose a primary conductor, on which an electrical connector is installed. The exposed components of the cable must be protected. For high voltage cables, such protection typically requires electrical stress control at the termination and various stress control elements have, been provided for this purpose.
Two types of polymer insulated transmission cable in wide use are ethylene propylene rubber (EPR) insulated cables and cross-linked polyethylene (XLPE) insulated cables. In each type of cable, a primary conductor is surrounded by a layer of the insulation (EPR or XLPE), which is in turn surrounded by a semiconductive layer. The semiconductive layer is a layer of a polymer composite including an electrically conductive material (e.g., polyethylene (PE) containing carbon black).
The end of the cable's semiconductive layer is the electrically most stressed location within a cable accessory. Components provided by accessory manufacturers are designed to reduce this stress to a tolerable level. However, in order to achieve this, certain requirements apply, which are difficult to meet with typically used cable preparation tools on EPR cables and their semiconductive layers. Namely, the region of the transition should be void free (no air pockets) and smooth to allow the stress grading components to follow the cable surface closely and provide sufficient interface pressure.
EPR cables usually have semiconductive layers that cannot be taken off using cutting type cable strippers designed for bonded semiconductive layers on XLPE cables. The semiconductive layers on EPR cables are typically strippable after applying moderate heat to the semiconductive layer (i.e., with a torch). Typical tools to define the end of the semiconductive layer to be stripped are cutting blades and round files. Blades may cut too deeply or not deeply enough considering the typical manufacturing tolerances of semiconductive layers. Cutting too deep may introduce immediate air pockets within the cut. Not cutting deep enough may cause air pockets by lifting up the end of the semiconductive layer when stripping the other end. Round files offer the advantage of providing a kind of chamfer at the cut back and a visual indication when the right depth has been reached. However, due to the tangential movement of the file, the risk of lifting up the remaining thin end of the semiconductive layer remains.
To provide a smooth transition from the cable's semiconductive layer to its insulation, conductive paints are sometimes used. However, many such paints do not stick very well to EPR even though they stick quite well to XLPE.